The present invention relates to a polyester resin composition having excellent transparency, gas-barrier property and whitening resistance upon absorbing moisture, which comprises, as main components, a polyamide resin produced from a diamine component containing 70 mol % or more of m-xylylenediamine and a dicarboxylic acid component containing 70 mol % or more of adipic acid, and a polyester resin produced from a dicarboxylic acid component containing 70 mol % or more of an aromatic dicarboxylic acid and a diol component containing 70 mol % or more of an aliphatic diol. The present invention further relates to films, sheets and thin-wall hollow containers produced by molding the polyester resin composition.
Polyester resins produced from a dicarboxylic acid component composed mainly of an aromatic dicarboxylic acid and a diol component composed mainly of an aliphatic diol (hereinafter may be referred to as "aromatic polyester resin"), typically exemplified by polyethylene terephthalate, have been extensively used as packaging or wrapping materials such as films, sheets and hollow containers due to their excellent mechanical properties, melt stability, solvent resistance, fragrance retention and recyclability. However, since the gas-barrier property against oxygen, carbon dioxide, etc. is not necessarily sufficient, the use of known aromatic polyester resins in applications requiring a high gas-barrier property is limited. To improve the gas-barrier property of the known aromatic polyester resins, there have been proposed a method of laminating foils of metals such as aluminum, a method of coating or laminating another resin film having a high gas-barrier property and a method of depositing aluminum or silicon. However, these proposed methods involve disadvantages such as deterioration in transparency, need for complicated production process, deterioration in mechanical properties, etc.
To achieve high gas-barrier property without needing complicated production process, there has been proposed a method of simply blending the polyester resins with another resin having a high gas-barrier property. Polyamide resins such as nylon 6 and nylon 66, especially those produced by polymerization of m-xylylenediamine and adipic acid (hereinafter may be referred to as "polyamide MXD6") are known as high gas-barrier resins. Gas-barrier resins other than polyamide resins include ethylene-vinyl alcohol copolymer resins. However, an aromatic polyester resin composition blended with ethylene-vinyl alcohol copolymer resin has various disadvantages such as whitening due to its poor compatibility with the aromatic polyester resin, poor stretchability due to its high crystallizability, low heat stability, etc.
Polyamide MXD6 has a high gas-barrier property as well as an excellent heat stability at melting, and the glass transition temperature, melting point and crystallinity thereof are close to those of the aromatic polyester resins, especially polyethylene terephthalate. Therefore, polyamide MXD6 is advantageous because it is easily compatible with the aromatic polyester resins by melt mixing, it does not deteriorate mechanical properties and stretchability of the aromatic polyester resins, and it exhibits a high gas-barrier property. However, the use of a composition of aromatic polyester resin and polyamide resin such as polyamide MXD6 in applications requiring a high transparency is limited due to its insufficient transparency. In addition, a composition of aromatic polyester resin and polyamide MXD6 is likely to cause whitening when exposed to high-humidity atmosphere or contacted with water in amorphous non- or low-stretched conditions.
Japanese Patent Application Laid-Open No. 1-272660 discloses a composition comprising a mixture of polyamide resin and polyester resin blended with tetracarboxylic dianhydride. The proposed resin composition is different from the polyester resin composition of the present invention in their chemical compositions. Although the proposed composition is taught to improve mechanical properties of molded products usable as engineering plastics, the prior art is quite silent as to a resin composition suitable for the production of films, sheets and hollow containers having high transparency.
Japanese Patent Publication No. 6-2871 proposes the use a compound having epoxy group and acid anhydride group, which is quite different from the tricarboxylic acid and its anhydride used in the present invention, to compatibilize a thermoplastic polyester resin with a polyamide resin containing m-xylylene group in the main polymer chain.